The present invention relates to a method of making an endless image-forming medium starting from a strip of semi-crystalline support material which strip extends between a first and second end, wherein the first and second ends are brought together and fused to form an endless support. The fused parts are post-crystallized and an image-forming layer is applied to the support.
Such a method is known from the international patent application WO 03/028982 and can be used, for example, as described in this application for forming a photoconductor for use in a printer. In this method, a strip of semi-crystalline support material is used as starting material, i.e. a material which is partially crystalline and partially amorphous such as, for example, the semi-crystalline polyester described therein. In this method, the head edges of the ends of the strip of support material are positioned against one another. The two ends are then fused together forming a weld. In the known method, the strip is heated to a temperature above the melting temperature of the material from which the strip is formed using radiation at the required weld location. As a result the ends of the strip are fused together. However, after fusing, the support material is significantly amorphous and thus a weak weld is formed. Also, tension is built up in the endless support. To make the weld sufficiently stronger and to reduce the problem of tension, the weld is treated so that the amorphous material at least partially re-crystallizes. In this connection, it is not necessary to achieve the same degree or form of crystallization as that of the original starting material. In one embodiment, and for this purpose, the weld is heated to a temperature where it does not melt but where the molecules of the molten material still have sufficient freedom of movement to be oriented with respect to one another, whereby the support material post-crystallizes and obtains a higher degree of crystallization at the weld location. In another embodiment, directly after the fusion of the two ends, the weld is slowly cooled so that the melted amorphous material has the opportunity to crystallize.
If an image-forming layer is applied to the endless support obtained in this way, an endless image-forming medium can be obtained which has no loss of image-forming functionality at the weld location. The advantage of this is that during image formation, no consideration need be paid to the location of the weld.
An important disadvantage of the known process is that the efficiency is relatively low. Although it is possible to obtain image media which have the same functionality at the location of the fused parts (hereinafter referred to as “the weld” in this description) as at any other location of the belt, the majority of the image media, that is, up to some 70%, has been found to exhibit considerably deviating functionality at this location. This deviating functionality takes the form, for example, in the occurrence of a stripe in the image at the place corresponding to the weld. Although the reason for this is not completely clear, it appears to be connected with defects in the image-forming layer at the weld location.
The purpose of the present invention is to provide a method having better efficiency. Thus, according to the present invention, prior to the application of the image-forming layer, at least a part of the support containing the fused parts is stretched, and the stretched part of the support is heated to a temperature above the glass transition point of the support material.
It has been surprisingly found that thermal treatment of the endless support, during which at least a part of the belt around said weld is under tension, enables the efficiency of the method to be significantly improved. By the application of this method it has been found possible to reduce the loss to 20% or less. It has been found that this treatment of the support should take place before the actual image-forming layer is applied to the support. To obtain the effect of the present invention, it is not important how much time elapses between the treatment of the support and the application of the image-forming layer or whether there are additional process steps therebetween.
Moreover, the favorable effect of the present invention does not appear to be due to the removal of any tension built up in the weld by the recrystallization process. On the one hand, the above-mentioned international application teaches that any tension build-up can be avoided precisely by recrystallization. On the other hand, in the method according to the present invention it is important that the temperature at which the endless support should be after-treated is above the temperature at which the initial support material has its glass transition point. If there is any tension in the weld, it would be precisely expected that a temperature above the glass transition point of the recrystallized weld material, which is typically 5 to 10° C. lower than that of the starting support material, should be sufficient. Also, the glass transition point of the support material can be determined, for example, in a method as known from the handbook Thermal Analysis by Bernhard Wunderlich, 1990, page 101 et seq. In the light of the present invention, the term glass transition point does not mean one temperature but all temperatures in the range of the glass transition point (described by Wunderlich on page 101, line 18, as “range of the glass transition”). The present invention can be applied at a temperature above the start of the transition (referred to as “Tb” by Wunderlich). The range of the glass transition point can be determined at different cooling (or heating) rates. Preferably, a very low rate is used, for example 1° C./min, particularly using a differential scanning calorimeter (DSC).
It should also be noted that the tension applied need have only a minimum value. It has been found that the present invention can be successfully used if the endless support is stretched at a tension not equal to zero, i.e. greater than zero. It should also be noted that the present invention is not restricted to a photoconductive layer as the image-forming layer. In principle the invention can be successfully applied to obtain a support for any layer on which an image can be formed. Nor is the invention restricted to obtaining a weld using a heat source to fuse the two ends. In principle, any technique leading to a comparable result can be used in the present invention.
From U.S. Pat. Nos. 5,885,512 and 6,068,722 it is known to thermally treat an endless photoconductor having a weld, the photoconductor being kept at a certain tension. The after-treatment known from this is not aimed at obtaining a higher percentage of photoconductors which initially have a good image-forming functionality at the weld location, but to withstand mechanical ageing of the photoconductor at the weld location. The processes known from this propose to subject the photoconductor to thermal after-treatment as a whole, i.e. including the image-forming layer. This after-treatment is aimed at removing internal tensions forming due to the application of different layers to one another. The present invention has realized that this known method does not provide the required improvement in production efficiency.
From U.S. Pat. No. 6,232,028 there is also known a method in which a photoconductor is subjected to tension at least in respect of a part and its temperature at the same time temporarily increased. This patent states that it is advantageous to select the temperature of the after-treatment which is below the glass transition point of the support.
In one embodiment of the present invention, after heating above the glass transition point and before the application of the image-forming layer the support material is cooled to a temperature below the glass transition point of the support material. As a result, the new state obtained is consolidated and the endless support can be mechanically treated without having an adverse effect on the production process. The result is greater freedom in the production process. Thus a support can be temporarily maintained before the image-forming layer is actually applied.
In another embodiment, the entire support is stretched. This embodiment has the advantage that the tension required can be easily obtained, for example by stretching the support over one or more rollers. This avoids the need to grip the surface of the support in order to stretch it. This might cause soiling or damage of the surface and this can, in turn, affect the functionality of the required image-forming medium. Also, apart from reducing the incidence of damage or soiling of the support as described above, it appears possible to further improve the production efficiency using this embodiment. The reason for this is not completely clear.
In another embodiment, the support is stretched over a drum having a radius slightly greater than the length L of the strip divided by 2π. In this embodiment, the support is stretched over one drum only, which has a periphery somewhat greater than the length of the endless support, typically up to 1%, and in one embodiment up to 0.15% greater. As a result, the support is as it were stretched over the drum by itself. This is a simplification of the method and consequently gives less rise to production defects.
In a further embodiment, the support is heated to a temperature above the glass transition point by placing it in an oven and on the drum on which the support is applied. This method on the one hand has the advantage that heating can be carried out very simply. On the other hand there is the advantage that as a result of the expansion of the drum the tension in the support can increase. This creates the possibility of keeping the initial tension at a minimum when the support is applied to the drum. The advantage of this is that the application of the support to the drum can take place with simple means and the risk of tearing when applying the support, particularly at the weld location, is very restricted.
In one embodiment, the image-forming layer is applied in the form of a solution, whereafter the solvent is evaporated. It has been found that precisely in this embodiment the maximum increase in production efficiency can be obtained. The reason for this is not clear.
In one embodiment the image-forming layer comprises a metal layer applied to the surface of the endless support. It is precisely with an image-forming medium of this kind that a deviant image-forming functionality was obtained at the weld location when using a method as known from the prior art. By the application of the method according to the present invention this can be significantly obviated.
In one embodiment, a polyester is used as the support material. The advantage of this material is that it is very resistant to water vapour and organic solvents. It also appears to be very suitable for use in the present invention.
In another embodiment, MELINEX is used as support material. This is a biaxially oriented polyester (polyethylene terephthalate) film made by DuPont/Teijin. This film appears particularly suitable for use in the method according to the present invention.